The present invention relates in general to methods for creation of porous coatings and the treating of metal surfaces using the coatings to render them effective for adsorption applications. In particular, the present invention provides methods for making high aluminum zeolite coatings on aluminum and aluminum alloys.
High aluminum zeolite coatings on metals in general, and on aluminum and aluminum alloys in particular are useful for adsorption applications, such as in air conditioning and air separation devices. It is known that high conductivity metals, such as aluminum and aluminum alloys are preferred as heat exchanger parts (e.g., hot coils and chilled coils, and fins) in an air conditioning system. One way of improving the performance of an air conditioning device is to increase the heat transfer efficiency for both the cooling coil and heat rejection coil by coating the heat exchanging surfaces (i.e., aluminum or aluminum alloys) with a coating that can selectively adsorb moisture from air. It is generally known that high aluminum zeolites including X, Y, A and many others are useful as adsorbents for moisture adsorption.
In addition to enhancing the efficiency of air conditioning applications, zeolite coatings may also be used in space applications for removing contaminants and for producing oxygen enriched air. For these applications as well as many others, aluminum or aluminum alloy substrates are preferred for their better thermal conductivity and lighter weight. Likewise, for these applications, high aluminum zeolites are preferred for their better adsorption properties.
Zeolite coatings can be deposited on aluminum and aluminum alloys by wash or dip-coating method, a method widely used for preparing heterogeneous catalysts on monolithic supports (e.g., automotive 3-way exhaust catalysts). A typical wash-coating process consists of slurry preparation (i.e., zeolite particles and a proper binder), slurry application by washing or dipping, air knifing, drying, and sintering. The wash-coating method has several significant limitations. For example, for very small wavy fins (e.g., 28 fins/inch), excessively strong air knifing tends to leave some fin surfaces uncoated while weak air knifing leads to clogged fins. Once the wet coating is formed, it has to be dried and sintered at high temperatures (e.g., 300–600° C.) to establish binding among the coating components and adhesion between the coating and the fin surface. However, this high temperature treatment is highly undesirable because it often adversely affects the mechanical properties of the underlying alloys. Other drawbacks of the wash coating method include poor adhesion, clogging of zeolite pores by the binder reducing their adsorption capacity and the taking up of the valuable volume and weight budget of the system by the binder which does not contribute to adsorption capacity.
An alternative to wash-coating method is in situ crystallization or its derivatives (e.g., seeded growth) that deposit zeolite coatings directly onto the substrate from a synthesis solution. This is a low temperature process (e.g., <200° C.), uses no binder and offers excellent adhesion. Current zeolite coatings synthesized by in situ crystallization on aluminum and its alloys include pure silica (no aluminum) and high silica (low aluminum) zeolite coatings. As used herein, pure silica zeolites have a silicon:aluminum ratio of infinity, high silica zeolites have a silicon:aluminum ratio greater than 100 and high aluminum zeolites have a silicon:aluminum ratio less than 5. Pure or high silica zeolites are hydrophobic, or not significantly hydrophilic, whereas, their aluminosilicate counterparts are hydrophilic. The pH of the synthesis solutions for many of the pure and high silica types of coatings is mild or neutral, allowing a zeolite coating to form on the aluminum before corrosion occurs. High aluminum zeolite coatings such as zeolite X, zeolite Y and zeolite A are characterized by high pH synthesis solutions, which upon immersion of the aluminum substrate, begin to oxidize and possibly totally corrode the substrate before a zeolite coating can form thereon. To date, there have been no reported high-aluminum zeolite coatings on aluminum and its alloys.